Numerous methods are known for removing SO.sub.2 from gas streams such as flue gas. These methods find particular application in the area of rendering gaseous effluents from industrial or utility power plants safe from an environmental standpoint.
A wide variety of absorbents have been heretofore utilized for absorbing SO.sub.2 from gas streams. The most advantageous processes are those wherein following absorption, the spent absorbent is capable of facile regeneration to its original state for further absorption of SO.sub.2. As a general rule, however, those compounds which are highly SO.sub.2 absorbent are difficult to regenerate; usually requiring a multi-step, expensive procedure for complete regeneration to the original absorbent state. Those compounds which, after absorption of SO.sub.2, are easily regenerable are normally not the most efficient absorbers of SO.sub.2.
The use of sodium salt solutions for absorption of SO.sub.2 have been in use for many years. The sodium compounds being soluble and more basic have obvious advantages over aluminum compounds. However, these very facts also work against sodium compounds, specifically during the regeneration step. For example, sodium bisulfite, the reaction product of SO.sub.2 scrubbing, is more stable and basic than either aluminum sulfite or aluminum basic sulfite and requires considerably more energy for regeneration.
Also, sodium sulfate which is formed during scrubbing is an irrevocable source of sodium loss. But in the aluminum system the scrubbing agent, i.e., aluminum hydroxide, is easily recovered from the sulfate. Moreover, the other products associated with Al.sub.2 (SO.sub.4).sub.3, for example, Al(OH)SO.sub.3 and Al(OH).sub.3, being insoluble, are easily separated from the soluble aluminum sulfate.
Aluminum sulfate also plays an important role in the regeneration step. On heating an aluminum sulfate solution the pH of the solution drops. But on cooling the pH of the solution reverts back almost to its original value. This unique property enables more facile removal of SO.sub.2 from the sulfite slurry.
In addition, the aluminum system can operate in the presence of chloride ions while in the sodium system chlorine equivalent of sodium is irrevocally lost.
Among the regenerative absorption processes heretofore employed is that described in U.S. Pat. No. 2,311,202 wherein an aqueous solution of basic aluminum sulfate is employed as the absorbent liquid. The spent absorbent is regenerated as an aluminum hydroxide precipitate which is redissolved in sulfurous acid and recycled for further absorption of SO.sub.2.
U.S. Pat. No. 3,943,230 also discloses utilizing basic aluminum sulfate as an absorbent for SO.sub.2.
U.S. Pat. No. 3,959,441 discloses a process for scrubbing sulfur dioxide from stack gases employing an absorbing solution prepared by adding a hydroxide of a Group III metal to an aqueous solution of a halide of the same metal.
U.S. Pat. No. 3,761,570 relates to the removal of sulfur compounds from gases by contact with an aqueous suspension of a hydrolyzed admixture of an oxide, hydrated oxide or hydroxide of aluminum, zinc, iron or manganese and a basic component such as an oxide or hydroxide of an alkali metal or alkaline earth metal.
It is an object of the present invention to provide a regenerable absorption process for removing SO.sub.2 from gases utilizing an aluminum compound as an absorbent for SO.sub.2, which aluminum compound has not been heretofore used in this capacity.